The Preserve by Patrick Lestewka

In the summer of 1967, seven men, members of an elite combat unit, embarked on a covert operation in the jungles of Vietnam. Two died. The survivors were forever changed.
Twenty years later, the remaining unit members receive a letter from an anonymous benefactor, along with a check for $50,000 and a promise of more to follow—if they agree to one final mission. Their task is simple: journey to the wilds of northern Canada to track down and kill three escaped convicts. The convicts are starving and unarmed. Easy money. A cakewalk.

But they have no idea what lays in wait on the snow-topped tundras and in the dark forests of the frozen north. Waiting with sharp teeth and slitted eyes and an old score to settle.

They have no idea.

But they will.

Soon.

They have entered...THE PRESERVE